1. Field of the Invention
This invention relates to a defect inspection apparatus and more particularly to a defect inspection apparatus which is suitable for inspection of liquid crystal substrates, printed circuit boards and photomasks used for manufacturing semiconductor devices.
2. Description of the Related Art
Semiconductor devices have been manufactured by exposing a circuit pattern that is formed on a photomask onto a silicon wafer using photolithography techniques. In this case, if a unallowable defect such as a pinhole or a pindot may exist on a mask pattern, the defect will be printed onto every wafer and will decrease yield of semiconductor devices. Therefore, it is crucial to detect critical defects on a photomask and repair the defects before exposing a photomask onto a wafer.
In order to detect defects of a mask pattern that are formed on a photomask, not only a die-to-die inspection method but also a die-to-database inspection method have been adopted. A die-to-database inspection method compares scanned image data obtained by optically scanning mask patterns and comparing the image with reference data obtained based on CAD (computer-aided design) data, and a die-to-die inspection method compares adjoining patterns sharing the same CAD data with each other. A die-to-die inspection method is easy to implement, whereas there is a risk that common defects that are located in the same spot of the adjoining patterns may fail to be detected. Since scanned image data is compared with CAD data in the die-to-database inspection method, a die-to-database inspection technique is more reliable, whereas it is a difficult task to conform the reference data to the scanned image data. As a result, difference between the reference data and the scanned image data may cause a false alarm and thus it is a difficult task to implement die-to-database inspection with high sensitivity.
As semiconductor devices become more and more integrated, a critical defect on a mask becomes smaller and smaller. It is required to enhance defect sensitivity. In order to enhance sensing sensitivity, it is necessary to generate high-fidelity reference image data to conform it to the scanned image data. In particular, major factors that may reduce inspection sensitivity includes aberration of an objective lens and an image lag of a time and integration (TDI) CCD sensor. However, it takes plenty of cost and time to design and fabricate a diffraction-limited objective lens at a wavelength of ultraviolet rays or deep ultraviolet rays in order to enhance modulation transfer function (MTF). Also, it is difficult to obtain image quality in an image of high signal-to-noise ratio. In particular, an image lag in a TDI sensor is partly due to recombination of an electron and a hole at a deep ultra-violet wavelength.
Further, methods for generating reference data have been researched and developed in order to enhance the degree of fidelity to the scanned image data. For example, optimization of optimal digital filter coefficients or the like based on scanned data is considered. However, conventionally, since the conventional modeling process is not complete because the process does not distinguish each factor such as aberration and an image lag, sufficient results have not been obtained.
Thus, in the conventional defect inspection apparatus, it is crucial to use the die-to-database inspection method in order to make a more reliable inspection, but it is difficult to enhance the degree of fidelity of the reference data to the scanned image data, thereby to reduce the inspection sensitivity. Therefore, it is desired to implement a defect inspection apparatus which enhance the degree of conformity of the reference data to the scanned image data, prevent occurrences of false defects, and make defect inspection more reliable by accurately modeling the characteristic of the scanned image data to generate the reference data.